When I did a fresh Funtoo 1.3 install on my desktop PC I just set my profile with epro and installed the KDE meta package and it worked (nearly) flawlessly. I did something like this:
#epro flavor desktop
# epro mix-ins kde-plasma-5-new
# epro mix-ins no-systemd
# epro mix-ins gfxcard-nvidia
# emerge -vauND --with-bdeps=y @world
# (say yes to any USE flag changes)
# dispatch-conf
# emerge -vauND --with-bdeps=y @world
#emerge -va xorg-server nvidia-drivers nvidia-settings
# emerge -va plasma-meta
# (say yes to any USE flag changes)
# dispatch-conf
# emerge -va plasma-meta
I have zero USE flags set in my make.conf and in package.use I only have what portage added.